1. Technical Field
The invention relates to a printing mechanism comprising a plurality of setting wheels which are mounted rotatably about a common axis and with which endless printing bands are in a drive connection which carry in a region of their length at the outer surface printing types and in another region of their length at the outer surface indicating types and which are led about the setting wheels and about a deflection edge arranged spaced therefrom and which are adjustable by turning the setting wheels, the printing bands being provided in the other region of their length at the inner face with teeth for engagement in corresponding recesses in the setting wheels, a setting shaft adapted to be brought into a drive connection with each of the setting wheels and carrying an actuating knob and stop means for limiting the turning angle of the setting wheels which comprise stop shoulders disposed in the path of movement of the tooth lying at the front in the respective adjustment direction of the printing bands.
2. The Prior Art
Such a printing mechanism is described in patent application No. P 34 06 762.0. When using the printing mechanism the printing types which are disposed at a predetermined printing position are inked in each case before the actual operation and then brought into contact with the medium to be imprinted. The display or indicating types are associated in their position with the printing types on the bands in such a manner that for each printing type which happens to be disposed in the printing position a corresponding indicating type is disposed in a display position and is visible to the operator so that the operator can see which character can be printed with the printing type at that instant in the printing position. For reasons of good legibility the indicating types are generally disposed on a white background. If the setting wheels could be turned with the aid of the setting shaft to any desired extent it could easily happen that the indicating types mistakenly reach the printing position and are inked with printing ink. It would then also be possible for printing ink to get onto the bright background of the indicating types so that the latter would then not easily be legible. The stop shoulders provided in this printing mechanism prevent in conjunction with the respective front teeth in the turning direction unintentional inking of the indicating types by limiting the turning angle of the setting wheels so that the indicating types cannot move into the printing position.
To make it impossible by forceful further turning of the setting wheels in the printing mechanism according to patent application No. P 35 06 762.0 for the printing bands to be adjusted to such an extent that the front tooth in the adjustment direction runs beyond the stop shoulder and indicating types undesirably reach the printing position, the actuating knob mounted on the setting shaft is coupled to the latter via a slip coupling. This slip coupling or clutch is so dimensioned that it slips, i.e. cannot transmit any higher torque to the setting shaft, after the front tooth in the adjustment direction has come to bear on the associated stop shoulder. The dimensioning of the slip coupling is a critical point. On the one hand, the actuating knob must be mounted so firmly on the adjusting shaft that it is completely impossible for it to slip too early, which would make normal turning of the printing bands for obtaining the desired setting operations no longer possible. On the other hand, it must engage lightly enough to ensure that it slips before the leading tooth in the adjustment direction can run past the stop shoulders, and this means that it must not slip too late. These contrasting requirements make a compromise necessary which leads to tolerance problems in dimensioning the slip coupling. In the printing mechanism described the slip coupling is formed in that the actuating knob is made from elastically deformable material and that the setting shaft is mounted in an axial bore in the actuating knob whose internal diameter in the condition detached from the setting shaft is less than the external diameter of the portion of the setting shaft intended for insertion into said bore. This gives tolerance problems in particular as regards keeping to the internal diameter of the bore of the actuating knob.
The invention is based on the problem of further developing a printing mechanism of the type outlined in such a manner that the problems occurring in conjunction with using a slip coupling are eliminated.